This invention relates to aqueous dispersions of polyamide resin, to modifying such resin dispersions by Michael addition with polyacrylates, and to curing acrylate-modified resin dispersions by exposure to radiation or thermal energy.
Polyamide resins are well known as a class of resins, as are numerous methods for their preparation. Polyamide resins are typically manufactured by reacting a di- or polyfunctional amine with a di- or polyfunctional acid. The properties of the polyamide resins will vary considerably depending upon the particular reactants employed in their synthesis. A known subclass of polyamides are aminoamide polymers having free (i.e., non-acylated) primary and secondary amino groups.
Polyamide resins are widely used in a variety of industrial applications. Polyamides are especially useful as adhesives and for forming water and solvent resistant coatings on substrates such as paper. An important related use of polyamide resins is as binders in printing inks and the like where film toughness, flexibility, and adhesion are important properties.
However, most polyamides are thermoplastic polymers which readily deform under heat and pressure and offer no practicable means by which they can be cured to form cross-linked, thermoset polymers. Cross-linked polyamides would be useful for many applications where properties such as solvent resistance are important. In addition, non-crosslinked, thermoplastic polyamides can be subject to cold flow, remelting, moisture degradation, and other modes of deterioration.
Unlike most polyamides, certain means are known for cross-linking aminoamides. However, such cross-linking generally requires a "two package" system, the aminoamide resin being one package, and a co-reactive resin--typically an epoxy resin--being the other package. Such "two package" systems are extensively used, but they require that two containers be employed and that considerable measuring and mixing be performed at the point of use. In addition, "two package" systems provide limited working time after mixing and generally require substantial time and/or heating to effect even partial cure.
One known approach to curing hot melt adhesives involves blending acrylate polymers into certain polyamides. However, the acrylate groups in these polymers have already been polymerized and are nonfunctional. Thus, the polyamides and acrylate polymers in such blends are not chemically linked by covalent bonds and cannot be cured or cross-linked to produce a thermoset material.
A number of classes of radiation curable acrylates are known to the art, such as the acrylates of polyurethanes described in U.S. Pat. No. 4,153,776 (Friedlander, et al.) and the acrylate derivatives of polycaprolactones as in U.S. Pat. No. 3,700,643 (Smith, et al.). However, these materials do not possess the adhesive character of the aminoamide resins. Moreover, they tend to be more costly than aminoamide resins made, for example, from tall oil.
It has been recognized that hot melt adhesives and other products based on polyamide resin present a number of problems relating to their application to various substrates. Common application methods involve heating the polyamide resins above their melting point and then applying the molten resins to the substrate. Such techniques, however, have certain inherent problems. For example, polyamide resins typically have high melting points, often higher than the thermal stability of the substrates onto which they are to be applied. Accordingly, the hot melt method can only be used in certain limited applications which require relatively expensive application equipment. Thus, the use of molten polyamide resins is not practical in many applications. Molten polyamide resins are also impractical where the resin is to be applied as a latent hot melt layer to be activated at a later time. For example, it may be desired to apply a polyamide resin to a textile interliner, incorporate the interliner into a garment, and then activate the adhesive to hold the assembled parts of the garment in position.
It has been recognized that certain of the application problems associated with the polyamide resins might be solved if the polyamides could be applied at ambient temperatures as a solution or a dispersion. For many applications, however, solutions of polyamide resins are unsatisfactory. Polyamide resins as a class have excellent resistance to solvents; even with respect to those solvents in which the polyamide resins are soluble, the solubility typically is relatively low. Furthermore, the solvents which have been used to make polyamide resin solutions often adversely react with the substrates to which the polyamide resin solutions are applied. A further problem associated with solvent solutions is that most solvents used are relatively expensive, often difficult or impossible to remove from the applied coatings, and present fire, toxicity, and environmental pollution problems.
To overcome or at least reduce the problems associated with such solvent solutions, it has been suggested to prepare emulsions or dispersions of the polyamide resins in water. Water is relatively inexpensive, evaporates fairly readily from applied coatings, is not flammable, and presents no environmental pollution problems.